speedometer correction problem

I have a problem with my speedometer and have had it since the day I bought the car two years ago. I have managed to live with it reading wrong, but I want to fix it if it's possible without too much work. Car is a 57 buick with dynaflow. It is reading about 67 at 55 mph. That is a guesstimate! I used a gps to calculate just how far off it was two years ago and I'm not positive that is an accurate reading according to my memory but close at any rate. My guess is possibly the speedo is off because the trans may have been replaced at some point. So another guess is that they never switch the speedo gear. I know!! a lot of guessing. LOL:D

Anyway, in order to switch the gear does the tail shaft have to be removed? Can it be removed without removing the whole drivetrain? Is there some sort of external adapter that can be adjusted to change the gearing for the difference in speed?

At least on my '54 with Dynaflow, you can inspect to see if you have the right speedo gear in the transmission easily.

Where the speedo cable attaches to the transmission there is a hex-headed fitting attached to the trans. This holds a small "driven gear" up against the larger "worm gear" inside.

On my car, at least, there is a number stamped on one hex side. Clean that all up with a wire brush, and start turning it out a bit at a time until you see the number, likely in the area of 18-20. This indicates the number of teeth in the driven gear. A look in the Buick parts book will tell you if it is correct for your differential gearing.

At least this is all the case for '54, but I would bet you will find the same...

This is great news! I will take a look.

I'd use the GPS to measure distance rather than speed. Reason is that the gear ratio of the drive gear and driven gear relate to not only the rear axle ratio but also to the tire size/diameter/revs per mile.

For example, if the GPS claims you've travelled .80 miles and your odometer claims 1.0 miles, THAT would indicate the incorrect gear(s) in the trans and/or a tire size issue (different in revs/mile than the orig equipment tire size).

As the speedometer cable is a dedicated, gear-driven situation, it rotates a "speed cup" on the back of the speedo head, which had a bar magnet inside of it. As the speed cup spins, it tries to spin another speed cup, which is attached to the speedometer display mechanism, working against a clockspring (so the display will return to lower speeds as the car slows down and/or stops).

The calibration of the gears works as mentioned, with the correct gear combination listed in the Buick parts book with respect to orig tire size.

The calibration of the speed display mechanism is done by varying the magnetism level of the bar magnet. The old-line speedo shops had a "zap" gun with which they'd adjust the magnetism level . . . a completely "trial and error, hit or miss, then hit again" situation until the best match of indicated speed was obtained across the normal driving speed range.

The "adapter" which the driven speedo gear works in has an offset hole, which allows for the different driven gear sizes (and related number of teeth). This allows for one adapter to be used with driven gears of different numbers of teeth, rather than having a number of "tooth-count-specific" adapters.

In times past, some mechanics had their own "rules" about how many teeth difference it took to change the indicated speed a certain amount, regardless of how accurate the odometer might have been to start with. Personally, I don't like that method as it can affect how you figure fuel economy . . . if that matters. Taking the extra effort to see what speedo gears are actually in the trans is a much better way to do things, even if it might take some extra effort to see what combination they might be.

Now, other than using the GPS, you can use a straight stretch (generally) of Interstate highway. What I used to do was to take a 10 mile run at 60mph (when that was a speed you could drive and not get run over or be too fast. As for the speed, 60mph = 60 seconds/mile. For about 5mph either way, 59 seconds = 61mph, and such. But the main thing is to check the odometer. Using a 10 mile run makes the math easier, but you can do it in 5 miles, too. Key thing is a constant speed, for good measure. Might also ensure that tire pressures are approx 30psi (cold), too. Some might claim the milemarkers aren't totally accurate, but I think that with a 10 mile run, things should average out pretty well.

So, check the odometer error first. That'll be a good first start toward isolating what your next steps might need to be.

Enjoy!

NTX5467

If it helps, attached is a photo of some I have accumulated, the color helps identify the number of teeth. Whatever you find in your transmission, lets say 20 teeth, you need to go to more teeth to slow the speedometer. The large gear on the end....I have no idea what it fits. I found one part number on a GM parts bag Group 4.337 part number 3987919 ... have no idea how many teeth on that number, Willis might have an old parts catalog that would tell.

Also there is a gear box made by Stewart -Warner. I have found them mostly on trucks. You can flip the gears and get over or under drive and I suspect there are different "sets" of gears that could be purchased. You might find these at swap meets or Ebay..... they were much more expensive than the plastic gears.

The "ratio adapter" which Barney pictured was used to fine tune the driven/drive gear ratio to better suit particular applications where just the two gears didn't provide the correct "cable speed" for accurate speedometer operation. They were on trucks and also many middle-'70s era cars. The stock ratio for the adapter is stamped on the case. In some cases, they were also used for an easier cable routing to clear other under-chassis components.

As Barney mentioned, the color code of the gear is related to the number of teeth on it. This made it easier for assembly line operatives to get the correct parts for each respective job. Please understand, though, that years of exposure to hot automatic transmission fluid can also add a reddish tint to the base color.

Enjoy!

NTX5467

is the odometer correct?

is the odometer correct?

I never did check the odometer, but it sure seems to role pretty fast. I will try and check this all out this week.

HOLD THAT SPEEDO GEAR CHANGE !!!

is the odometer correct?

My speedometer shows 30% - 35% faster than actual speed.

I checked it with my GPS nuvi. And while I was at it, I checked the odometer.

On a 200 mile round trip, the odometer was dead on accurate. According to GPS and the highway mileposts.

And the speedometer was consistently, about 1/3 faster than tbe speed clocked by GPS.

So, before you start changing gears, check the resistance in the speedo head.

Sounds like a problem with the magnets in the speedometer head. Take it to a speedometer shop and they should be able to fix it.

Sounds like a problem with the magnets in the speedometer head. Take it to a speedometer shop and they should be able to fix it.

On a 57 buick with dynaflow, it is not likely that the rear gears or tires changed to affect the speedometer that much. In the meantime drive with a GPS.

I agree.

I'm sure that it is the 59 year old magnet has lost some magnetism.

My 2003 Durango has tires two sizes larger than the specified tire size.

The larger tire makes about 5% error (again checking against GPS Garmin Nuvi).

I don't think a difference in tire size would show 67 at an actual 55

Not only might the magnetism strength change, but so can the tension on the clockspring that returns the indicator to "0". But magnetism adjustments can adjust for that, too.

Enjoy!

NTX5467

Well I refreshed my mind with doing the gps thing again. It was just as I remembered. First comparing the speedo with the GPS I was showing 65mph at the car speedo and 52 at the gps. A second take showed 80 at the car speedo and 65 ish at the gps. Also my odometer showed about 4 tenths off at 7 miles clocking against the mile markers. I'd like to do a mile for mile comparison, but didn't quite have the time today. My tires are larger than what the car was equipt with, but when I raised this question before I was told there was no way the tire size had much to do with the difference in speed. I'm still leaning a bit towards changing the gear for a different size.

Your odometer is 6% in error.

Your speedometer is 20% - 25% in error.

Tire size may be the culprit for the 6% error.

If you change the gear, a 6% reduction should be made.

I say your problem is the magnet (lack of) resistance.

Edited to correct mathematical error

Check my math ...

65 - 52 = 13

13/52 = 0.25 (or 25%)

0.4/7 = 0.057 (or 6%)

There is a shop on Dort hiway in Flint Mi,( Specmo ) that has a machine to run the car at speed and then then build a special conversion

gear that goes between the cable and the transmission to correct the speed. Had it done to my 40 41C a few years ago and to my knowledge

they are still in business. Jim Brayton Buicktown.

If the odometer is correct but the speedometer is wrong, changing gears will mess up the odometer as well. You might get the correct speed but your odometer will be really screwed up. Tire size, especially if you're running radials and there's no true conversion from bias to radial could amount to the odometer difference. It still sounds like the problem is in the speedometer head in the magnet. The odometer is a direct gear driven mechanism, any change in gear ratio will effect it. The speedometer is not directly gear driven. It's based on slippage on the magnet. Magnets wear out, the speedometer reads wrong. Gears are either good or bad, the odometer will work according to the gear ratio.

Just use the GPS for your speedometer, I do that on my new car because for some reason it doesn't read accurately at all speeds - the speedometer reads too slow below 60, at 60 it's right on, and over sixty it reads too slow. Plus it's digital and quick to read. just don't worry about the speedometer in your car.

As one of my old bosses used to tell me. "Don't sweat the petty stuff and don't pet the sweaty stuff."